Various systems use inductive coupling techniques for providing electrical power and communicating with miniature devices, such as medical implants and identification tags. For example, U.S. Pat. No. 5,105,190, whose disclosure is incorporated herein by reference, describes an electromagnetic identification system in which a transmitter generates an electromagnetic interrogation field in a detection zone. At least one responder is provided capable of disturbing the interrogation field. A detection circuit detects a disturbance of the interrogation field caused by a responder. The responder is a single receiver circuit which responds to the interrogation field which supplies energy to activate a digital encoding circuit having a binary-code defining circuit. The encoding circuit is arranged, in operation, through a switch to vary the electrical characteristics of the receiver circuit to generate a code signal that is detectable by the detection circuit. The encoding circuit is arranged to control the switch during the pre-determined periods of time for varying the electrical characteristics of the receiver circuit. A programmer is provided to modulate the transmitter to generate a modulated interrogation field.
U.S. Pat. No. 6,778,070, whose disclosure is incorporated herein by reference, describes a remote transmission apparatus for a portable object equipped with an antenna and connected by inductive coupling to a fixed station. A data processor processes and stores data concerning the portable object, a voltage rectifier transforms the AC voltage supplied by the antenna of the portable object into a DC voltage carried by a power line to the data processor, a voltage regulator regulates the DC voltage provided by the rectifier, and a modulator modulates the signals to be transmitted. The modulator comprises a transistor connected to an electronic dipole that causes a drop in voltage between the input and output of the modulator, the modulator being connected to the input of the voltage regulator to control the voltage on the terminals of the antenna in order to modulate the current flowing in the antenna.
U.S. Pat. No. 8,285,204, whose disclosure is incorporated herein by reference, describes a contactless data transmission device that includes an antenna circuit supplying an alternating voltage in the presence of an alternating excitation field, a charge modulation transistor having a control terminal receiving a charge modulation signal having at least one low state and one high state, and a conduction terminal receiving the alternating voltage. The device includes a circuit for supplying a boosted direct voltage greater than a peak value of the alternating voltage supplied by the antenna circuit, and a circuit for applying to the control terminal of the charge modulation transistor a charge modulation signal which, in the high state, has a voltage level equal to the boosted direct voltage.